Refined crude or synthetic oils or, compositions which include refined crude or synthetic oils, released into the environment are toxic and/ or detrimental to the environment and are often used in industry with negative effects to the environment, for example, drilling fluids (sometimes called muds) used in offshore oil and gas production and exploration. Drilling fluids are used to lubricate the drill bit and to carry the debris, such as drill cuttings, up to the surface for disposal. The debris is normally separated from the drilling fluids, however, the debris retain a layer of the drilling fluid. The oil covered debris resulting from such well boring operations need to be shipped to land for safe disposal or, if it were to be discharged onto the seabed or overboard into the sea, it needs to comply with strict environmental impact restrictions. Due to the high expense of shipping and disposing of the mud drilling compositions, a need exists to use drilling fluid which can be discharged onto the seabed or overboard and which complies with the strict environmental impact restrictions. One such requirement is the Environmental Protection Agency (EPA) LC50 requirement of more than 30 000 result in a Mysid shrimp (Mysidopsis Bahia) bioassay prescribed in 1984 EPA-600/3-84-067. Generally, the Mysid shrimp bioassay measures the toxicity of the water column in which the shrimps live. Recently it became apparent that not only is the toxicity of the water column relevant, but even more so is the toxicity of the seabed sediment, onto which discharged debris settle after it has been discharged overboard. Therefore, more relevant for drilling fluids used where the debris is to be discharged overboard, is the requirement that oils used for the manufacture of such drilling fluids pass a stringent ten day marine amphipod (Leptocheirus plumulosus) acute sediment toxicity test in accordance with American Society for Testing and Materials (ASTM) Guideline E 1367, EPA 600/R-94/025, which tests the toxicity of the actual marine sediment. This test is especially relevant to all offshore drilling platforms since the discharge of toxic drilling mud compositions from a drilling platform onto the seabed would have a significant negative environmental impact on the seabed.
In this specification, the hydrocarbons will be understood to be a collective term for molecules comprising carbon and hydrogen only and include non cyclic saturated hydrocarbons referred to as “paraffins”, unsaturated hydrocarbons referred to as “olefins”, cyclic hydrocarbons referred to as “cycloparaffins” and aromatic hydrocarbons referred to as “aromatics”. Straight chain paraffins will be referred to as n-paraffins and branched paraffins referred to as iso-paraffins. Synthetic hydrocarbons will be understood to mean any hydrocarbons derived from a chemical process in which a chemical reaction takes place, as opposed to natural hydrocarbons, which is refined or distilled from crude oil.
Natural hydrocarbons, which are refined or distilled from crude oil are normally contaminated with high levels aromatics and are relatively toxic to marine life, making these drilling fluids that contain “natural” crude hydrocarbons environmentally unacceptable. High levels of n-paraffins in these fluids would have poor cold flow characteristics limiting their application in cold environments due to formation of waxy deposits.
The process for the preparation and use of plant or vegetable oil based environmentally friendly drilling fluid has been described in U.S. Pat. No. 4,631,136.
The use of synthetic hydrocarbons became popular due to their low aromatic content and availability. Several patents described the use of synthetic hydrocarbons for drilling fluids. U.S. Pat. No. 5,096,883 discloses the use of C18 to C40 hydrocarbons derived from dimerised 1-decene which is esterified. The good biodegradability of esters is well known, but esters are hydrolytically unstable. U.S. Pat. No. 5,589,442 discloses the use of non alpha, linear internal C14 to C18 olefins obtained by an alpha olefin isomerisation process. U.S. Pat. No. 5,569,642 discloses the use of a preferable C14 to C20 blend of n-paraffins and iso-paraffins. This patent also teaches that iso-paraffins having up to 40 carbon atoms per molecule are liquids over the temperature range of interest for drilling fluids, whereas, n-paraffins having more than about 16 to 23 carbon atoms per molecule are waxy solids. This is important with regard to the viscosity and rheology of drilling fluids. Similarly, U.S. Pat. No. 5,866,748 discloses the use of a mixture of C8 to C20 n-paraffins and iso-paraffins derived from hydro isomerisation of C8 to C20 n-paraffins. U.S. Pat. No. 6,096,690 discloses, by way of example, the use of a mixture of C13 to C18 n-paraffins and iso-paraffins derived from hydro cracking of Fisher Tropsch waxes. This patent further claims that mono methyl iso-paraffins are less toxic than more branched iso-paraffins. U.S. Pat. No. 5,498,596 discloses the use of a mixture of C10 to C18 paraffins from mineral oils and poly(alpha olefins) derived from the dimer of decene. U.S. Pat. No. 5,189,012 and a related US Registered Statutory Invention No. H1000 discloses the use of branched chain oligomers and unhydrogenated synthetic hydrocarbon compositions of C9 to C71 synthesized from oligomerization of C2 to C14 olefins. U.S. Pat. No. 5,635,457 discloses, in one embodiment, the use of a hydrocarbon mixture of which at least 95% has 11 or more carbon atoms and, in another embodiment, at least 95% has 10 or more carbon atoms.
Each of the above patents utilised a water column toxicity bioassay. All, except U.S. Pat. No. 5,498,596 which used a marine Copepod bioassay, used the Mysid shrimp bioassay.
The applicant has found that, for the (Leptocheirus plumulosus) acute sediment toxicity test, the toxicity rapidly decreases for a distillation fraction of hydrocarbons the higher its boiling point above about 270° C. A trend in toxicity reduction was noted for toxicity as the boiling range of the fluid increased.